1. Field of the Invention
This invention relates generally to information storage discs and, more particularly, to apparatus for rotating an information disc relative to a radially movable transducer.
2. Description of the Prior Art
Discs for storing large quantities of video information have come into increasing usage in recent years as a result of an increasing need for storage media that provide instantaneous playback, fast random access, and relatively high recording density. The information is typically encoded on the disc in the form of an optically readable sequence of light-reflective and light-scattering regions arranged in substantially circular tracks forming a spiral or concentric circular pattern over the information-bearing surface of the disc.
Ordinarily, the light-reflective and light-scattering regions are initially formed in the disc using an optical transducer for directing onto the disc a collimated beam of high intensity light that is modulated by the information to be recorded. The disc is rotated about its central axis at a substantially constant angular velocity relative to the transducer, while the beam of light is moved radially with respect to the disc at a relatively slow, but constant, velocity. Each revolution of the disc thus results in the production of a separate information track. In the case of video signals, the disc is ordinarily rotated at approximately 1800 r.p.m., whereby each information track contains the information for one video frame.
Systems for encoding the discs at a constant angular velocity have not utilized all of the information storing capability of the discs, however, because the successive light-reflective and light-scattering regions forming information tracks near the periphery of discs are significantly larger in size than the corresponding regions forming tracks near the centers of the discs. As a result, the density of the recorded information is substantially less at the periphery of the disc than near the center of the disc.
Efforts have been made in the past to obtain a more uniform distribution of information over the surface area of the disc, by reducing the angular velocity of the disc as the radial position of the light beam increases. A uniform density of information can be produced if the angular velocity of the disc is made inversely proportional to the radius of the particular track being recorded, whereby the track being recorded on is moved at a constant linear velocity relative to the beam. If this is done, the nominal sizes of the successive light-reflective and light-scattering regions are the same for all of the information tracks.
An example of an analog system for rotating an information disc at a constant linear velocity relative to a beam of light is disclosed in a copending application for U.S. patent application Ser. No. 961,405, filed simultaneously herewith under the name W. R. Dakin et al and assigned to the same assignee as the present application, and entitled "Method and Apparatus for Rotating an Information Storage Disc". The disclosed system includes a movable lens carriage for directing a light beam onto a disc, with an associated potentiometer for producing a voltage signal proportional to the radius of the particular information track being recorded. This voltage signal is applied to a voltage-controlled-oscillator (VCO) to produce a disc velocity signal having a frequency proportional to the radius of the selected track. The velocity signal is, in turn, coupled to a spindle servo, which includes a spindle motor and an associated tachometer, for controllably synchronizing the frequency of the signal produced by the tachometer with the frequency of the velocity signal, whereby an angular velocity substantially proportional to the radius of the track being recorded will result.
Although constant linear velocity systems of the type described above do provide a more uniform density of recorded information over the surface of the disc than do constant angular velocity systems, and although they operate satisfactory in some situations, the systems are subject to non-linearities and drifts in the VCO along with inaccuracies in the voltage signal produced in the variable resistor. The density of the recorded information thus cannot be made as uniformly high as it would be if the system were not susceptible to such errors. Additionally, such constant linear velocity systems ordinarily require elaborate calibration procedures to be performed prior to their actual use.
Other systems for rotating an information disc at a substantially constant linear velocity relative to a light beam have eliminated the potentiometer and substituted for it a shaft encoder coupled to the movable lens carriage, a counter, and a digital-to-analog (D/A) converter. In these alternative systems, pulses produced by the shaft encoder are counted in the counter to produce a digital count proportional to the radius of the track being recorded. Application of this count to the input of the D/A converter, then, produces a corresponding analog signal than can be used in combination with a VCO and a spindle servo of the type previously described, to control the angular velocity of the disc. Such hybrid digital/analog systems still require elaborate calibrating procedures to be performed, and are still susceptible to non-linearities and drifts in the VCO. Additionally, such systems are limited by the resolution capability of D/A converters currently available commercially.
It will be appreciated from the foregoing that there is still a need for a method and apparatus for rotating an information disc at a precisely controllable angular velocity, whereby information can be recorded in a series of concentrically arranged tracks with a uniformly high recording density, without being susceptible to non-linearities and drifts of various elements in the apparatus, and without necessitating substantial initial calibration procedures. The present invention fulfills this need.